Diamond can be said to be a specific crystal lattice in which a color center in a crystal behaves like so called “atoms (trapped atoms) in low temperature and a vacuum” in room temperature and the atmosphere. A nitrogen-vacancy complex (NV center) that is formed in the specific crystal lattice, or diamond, is one type of color center, and involves nitrogen (N) substituted with carbon and an atomic vacancy (V) that is positioned adjacent to this nitrogen, as is illustrated in FIG. 1, and has a spin S=1.
An NV− center, which is the NV center in a negatively charged state once it captures an electron, is only one solid characterized by the fact that a single spin can be operated and detected by light at room temperature and the coherence time is long, and is expected to be applied to a magnetic sensor having high spatial resolution and high sensitivity (see Non-Patent Literature 1 by D Le Sage et al., Non-Patent Literature 2 by J. R. Maze et al., and the like). There is a report that a magnetic detection limit of the sensor using the NV− center at ordinary temperature largely exceeds detection limits of a hole sensor and an impedance sensor, and is equivalent to the detection limit of SQUID, on a theoretical calculation (see Non-Patent Literature 3 by V. M. Acosta et al.).
FIG. 2 is a view for describing a principle of magnetic detection using the NV− center. The NV− center can take three electron spin states (triplet state) of |0>, |1> or |−1>, in a ground state. In the figure, Δ represents an energy difference between the |0> state and the |±1> state, γ represents a magnetic rotation ratio, and B represents magnetic field strength.
When the NV− center in the ground state is irradiated with green light, the NV− center emits red fluoresce. However, when it has the electron spin of |1> or |−1> in the ground state, some of the electrons after excitation pass through a singlet state and returns to the ground state, which makes it difficult to cause a fluorescence process. Energy separation (2γB) between these electron spin states of |1> and |−1> is proportional to the magnetic field strength B, and accordingly when a sensor using the NV− center is irradiated with a microwave having a frequency of approximately 2.8 GHz, and the frequency of the microwave is swept, the sensor can detect the magnetic field strength as a brightness lowering point of the red fluorescence.
FIG. 3 is a view for conceptually describing that the brightness lowering point of the red fluorescence at the time when the frequency of the microwave has been swept varies depending on the magnetic field strength. This view, in which a horizontal axis is the frequency (GHz) of the microwave and a vertical axis is the brightness of the red fluorescence (arbitrary scale), conceptually shows that when the magnetic field B is changed in a range of 0 to 12 GHz, a split (Δf) between the frequencies (f1 and f2) of the microwave at the brightness lowering point of the red fluorescence increases proportionally to the magnetic field strength.
On the basis of such a principle, there is a reported result of measurement of a two-dimensional distribution of a weak magnetic field of approximately 1 mT (see Non-Patent Literature 4 by S. Hong), and there is also a report that a magnetic field of an fT level can be measured in principle (see Non-Patent Literature 3).